1. Field of the Invention
This invention relates to optical systems, and more specifically, to a scatterometer-interferometer optical inspection head and system and method for measuring surface topology and for detecting and distinguishing characteristics of surface defects.
2. Background of the Invention
Optical surface inspection systems are in common use in industry to determine whether surface features are present on an article as desired, and further whether undesirable defects or contaminants are also present.
Scatterometers are well-known and commonly used in surface inspection systems to determine whether defects are present. Since scatterometers typically operate in dark-field mode, they are very sensitive to the sharp increase in intensity due to the presence of a defect, feature or contaminant on a surface. Scatterometers are also tolerant of variation in source intensity and to some degree, source alignment. However, scatterometers do not typically yield much information about a surface artifact other than its location and presence. Hence, scatterometers are typically not used when information about the size or nature of surface artifacts must be determined. Due to their dark-field detection, scatterometers can typically employ very sensitive and fast detectors such as photo-multipliers and can use very large “spot” sizes with high-intensity source illumination in order to detect small defects very quickly. Scatterometers are typically used in continuous scanning mode with a threshold on the detected reflection value(s) indicating only the presence of a defect within the spot.
Various differential methods have been used to single out artifacts on a surface, such as optical lever methods that measure local surface inclination, phase-contrast or differential interference contrast (DIC) microscopy. However, the differential methods mentioned above are not sensitive to defects that are too smooth to produce an appreciable difference. In addition, the above-described systems give only a relative indication that must be mathematically integrated in order to produce a full surface profile, a procedure that is mathematically error-prone.
Height measuring interferometry is the method of choice for characterization of surface topography including measurement of gross surface features such as inclination and curvature, as well as measuring individual surface artifacts. Of particular usefulness are interferometric methods that provide local height measurement using a single spot, as a direct measurement of an artifact can be made and such systems are amenable to scanning large surfaces. Such systems not only can measure topographic features or defects that only slightly scatter incident light, but can also distinguish height from depth (bump vs. pit defects), as well as providing actual height or depth magnitudes along with the lateral size of the features. For artifacts smaller than the spot size, although the interferometric signal combines both the height and size information so that the reported height/depth is typically smaller than the actual vertical dimension, true height and size information can be recovered by oversampling and deconvolution techniques well known in art.
Single-spot interferometers commonly used for height measurement include Michelson, fringe-counting, phase-shifting, and Doppler types. The advantage of the height measuring single-spot interferometers over other systems is that direct sampling of height is provided. Two-spot (differential) interferometers measure the height difference between neighboring spots, which will always include the local inclination angle in the direction of offset between the spots. In particular, surface topography is given directly in single-spot techniques whereas in differential techniques, the topography must be reconstructed by integration.
Recently, resonator-enhanced optical measurement systems have been introduced as disclosed in U.S. Pat. Nos. 6,714,295 and 6,927,864 and 7,022,978, the disclosures of which are incorporated herein by reference. The incorporation of a resonator in the inspection system greatly increases the sensitivity and/or resolution of the inspection system, so that smaller features and defects can be detected and information gathered about their size, height and properties. Resonator-enhanced interferometric systems have an even smaller spot size and therefore require even more sampling, and hence more computation, to produce accurate defect detection.
There are several drawbacks to interferometric height measurement for defect detection. Interferometric measurement is intrinsically a discrete (sampled) measurement rather than being continuous and defects that are small or have shallow profiles produce small signals close to the detection threshold may be missed. Also, most interferometers are inherently bright-field measurement systems, and are therefore sensitive to variations in source intensity and variations in surface reflectivity, which also may lead to missed defects and/or false triggers.
Therefore, it would be desirable to supplement an interferometric height measurement system that provides detailed information about the size and/or properties of a surface artifact with fast and sensitive artifact detection. It would further be desirable to provide such surface artifact detection in a resonator-enhanced interferometric measurement system such as the Fabry-Perot resonator-enhanced systems disclosed in the above-incorporated U.S. Patents.